Phone System is Microsoft's technology for enabling call control and Private Branch Exchange (PBX) capabilities in the Microsoft 365 cloud with Microsoft Teams. Phone System works with Teams clients and certified devices. Phone System allows you to replace your existing PBX system with a set of features directly delivered from Microsoft 365.
To connect Phone System to the PSTN, you can choose from the following options:
Calling Plans are quick and easy but don't represent great value as you scale, and the calling tariffs aren't great.
Operator Connect is fine as long as you're able to accept the limits imposed by the carrier. There was after all a reason that PBXs became popular.
Direct Routing offers the most flexibility but is the most complex, this allows you to maintain your existing arrangements, including your PBX
Others via plugin applications, bots, etc other options are available but these normally break the Teams experience.
ACME UC offer calling plans via CSP, Direct Routing solutions and options as well as fixed all in, per month contracts suitable for smaller businesses.